The bound states for the hydrogen atom are degenerate. For a given n, the allowed l values are l = 0, 1, ..., n-1. Including all the l and m values for a given n leads to a degeneracy of n². It is common to refer to the l=0, 1, and 2 states as s, p, and d orbitals, respectively. For atoms with more than one electron, the Pauli exclusion principle states that two electrons can not occupy the same state. Thus, for example, helium has one electron in the 1s spin-up orbital, and one electron in the 1s spin-down orbital. In this manner, the periodic table of the elements is created by filling the lowest energy levels first. To have stable solids, i.e., have the atoms stick together, one needs the electrostatic force, the Schrodinger wave equation, and the exclusion principle.